The present invention relates generally to power supplies and, more particularly, to single-power-supply circuitry for providing gridding and biasing functions suitable, for example, for electron tube applications, such as x-ray tubes or electron beam guns.
In one application, gridding circuits are employed for controlling x-ray tubes in pulsed fluoroscopy in order to reduce radiation exposure to patients resulting from decaying x-ray tube current. In particular, x-ray tube gridding involves rapid application and removal of a high-voltage (in the range of a few kilovolts) to a cathode cup, the cup functioning as a grid in conventional electron tubes. Upon grid voltage turn-off, the x-ray beam is turned on, and vice versa. The length of the on and off periods are typically measured in tens of milliseconds. High-voltage isolation is required for such applications, as the gridding circuit is referenced to a cathode potential of up to -70 kV in standard tubes and up to -140 kV in grounded anode tube designs.
In addition to grid control, an x-ray tube also requires bias control, i.e, for controlling the focal spot size of the x-ray beam. Hence, a grid-controlled x-ray tube typically has two control electrodes for providing the gridding and biasing voltages. Typically, two separate power supplies are used to provide the two voltages.
Accordingly, it is desirable to provide a single power supply circuit capable of providing both the gridding and biasing functions. To be practicable, it is further desirable that such circuitry be relatively simple, small in size, and reliable.